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Passivation of Stainless Steel

Plating Guidelines

Black Nitride || Cadmium Platings || Dry Film Lubricants || Electro-Tin/Solderable Platings || Nickel Platings || Passivation of Stainless Steel || Zinc Platings

EXPLANATION OF TYPICAL PASSIVATION PROCEDURES

Passivation is the generic term applied to procedures developed to remove the presence of metallic (free) iron from the surface of stainless steel products. The presence of free iron particles on the surface of stainless steel products can cause the product to show rust spots which should not occur with a clean or “passive” stainless steel surface. A PASSIVE stainless steel surface would have NO free iron particles which could potentially corrode or “rust.”

During the handling and processing operations such as forming, machining and tumbling, particles of iron and tool steel can be embedded or smeared on the surface of the fastener. If allowed to remain, these free iron particles will corrode and show as rust spots.

The passivation process involves submerging the stainless steel fastener into a solution of nitric acid (20-25% by volume), sodium bicarbonate (2-3% by weight) and water or 35% Nitric by volume and water for some period of time. Twenty to thirty minutes is typical. The nitric acid dissolves the embedded or smeared iron and restores the original corrosion-resistant surface to the part.

It is generally considered good practice to passivate parts after many manufacturing processes, however in some applications it may prove unnecessary. When the need for passivation is in doubt, testing can provide the answer. ASTM A380 is a recognized specification which details the specific testing parameters to be met. Parts which pass the “Passivation Test” per ASTM A380 have a surface void of any free iron particles. They are considered passive.

EXPLANATION WITH REGARD TO PEM FASTENERS

Penn Engineering typically lists Passivated and/or Tested per ASTM A380 as it’s passivation procedure. ALL parts are subjected either to a passivation procedure OR are tested for the need for passivation. Those parts which FAIL the passivation test are then passivated. Those passing the test are passive and are not processed further.

For those customers requiring that all parts are passivated and not just tested, a special part number can be assigned and ordered.

APPLICABLE INDUSTRY SPECIFICATIONS

  • PASSIVATION per QQ-P-35C, TYPE II or it’s replacement ASTM A967 Nitric I, used as the processing spec. for passivating AISI 303, 410, 416 and 17-4PH stainless steels. Most PEM fasteners manufactured by screw machining methods are passivated to this specification when necessary.
  • PASSIVATION per QQ-P-35C, TYPE VI or its replacement ASTM A967, Nitric 2, Used as the processing spec. for passivating AISI 302, 304, 305, 316, 900 stainless steel. Most PEM fasteners manufactured by cold-heading or cold-forming methods are passivated to this specification when necessary.
  • PASSIVATION test per ASTM A380 Used for determining if a product needs to be passivated. Detects the presence of free iron on the surface. Those products failing this test are then passivated by one of the above specifications.

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